To test what the inherent angular noise of the HeNe 1103P laser is, we're testing it on a table pointing into the BS OL QPD with only a few steering mirrors.

From the setup that I found today, I've removed the lens nearest to the laser (which was used for the BS and PRM) as well as the ND filter (what was this for?) and the lens placed just before the BS QPD.

With the ND filter removed, the quadrant signals are now ~15000 if we misalign it and ~9000 each with the beam centered.

In order to calibrate the OLPIT_IN1 and OLYAW_IN1 signals into mm of beam motion, I misaligned the mirror just before the QPD. The knobs on there actuate the 100 TPI screws and the knurling on the knob itself has 10 ridges, so that's 36 deg per bump.

PIT cal ~ 1.55 (knob deg / count) -->> 10 microns / count --->>> 10 urad / count

YAW cal ~ 1 (knob deg / count)  -->> 6.5 microns / count --->>> 6.5 urad / count

Distance from the 45 deg turning mirror to the QPD silicon surface is 23 cm. Distance between knob tip and fixed pivot point is ~4 cm. 1 knob turn = 0.01" = 0.254 mm = 0.254/40 radians of mirror angle.

So 360 deg of knob gives 2*0.254/40 = 0.012 radians of beam angle = 0.012 * 230 mm ~2.3 mm of beam spot motion. Or 6.4 microns of translation / deg of knob.

The distance from the face of the laser to the QPD is 96 cm.

The punchline is that the laser shows a level of noise which has a similar shape to what's seen at LLO, but 10x lower.

The noise at 0.05 - 0.2 Hz is ~2-3x worse than the PR3 at LLO. Not sure if this is inherent to the HeNe or the wind in our setup.